Controllable pitch marine propellers provide numerous advantages over fixed pitch propellers, especially in vessels that operate at various speeds with varying loads. A well-known form of controllable pitch propeller is the force rod type, in which a blade pitch control mechanism in the propeller hub is operated by a force rod that extends through the shaft and is moved forward and aftward by an inboard hydraulic servo or other suitable means. The pitch-control mechanism in the hub comprises a cross-head affixed to the aft end of the force rod and coupled to each of the blades by crank pins and slideways. In some designs the slideways are on the cross-head and the pins on the blade mounts. In other designs the pins are on the cross-head and work in corresponding slideways on the blade mounts. The blades are, of course, mounted on the hub for rotation about pivot axes disposed radially of the propeller shaft axis, and the crank pins and slideways are offset generally circumferentially from the pivot axes of the blades.
In the majority of present designs of force rod type controllable pitch propellers, the cross-head is generally a block of metal attached to the end of the force rod and containing slideways for each crank pin sliding shoe. It is located within the propeller hub such that at a neutral (zero pitch) position it lies radially inwardly of the blade-mounting trunions. Upon movements for ahead and astern pitch control the cross-head moves forward and aftward from the neutral position. With this configuration, the open region of the hub swept by the cross-head in its pitch-controlling motions has to be aftward of the aft end of the shaft. Accordingly, the shafting and aftmost shaft bearing have to be designed to carry a substantial overhung moment due to the load of the propeller, which is located almost entirely aftward of the aft end of the shaft.